Hey, what happens when you have code like ```fun foo(): Either<Err, Ok> val res = foo().map { ...

# arrow

[Kristian Nedrevold]

Hey, what happens when you have code like

fun foo(): Either<Err, Ok>
val res = foo().map { bar(it) }.mapLeft { return it }

and foo() is Err?

[Kristian Nedrevold]

val res = bar(foo().getOrHandle { return it })

Does what I want here I think?

[qohatpp]

val res = foo().map { bar(it) }.mapLeft { return it }

1. if foo() is Right bar(it) is going to be executed and you will have a Right(bar(Ok)) as result, which is an Either<Err, Whatever Your bar(Ok) type is>
2. if foo() is Left then your mapLeft is gonna be executed giving a Left(Err) as result.
In your example seems like mapLeft is not longer necessary because you are returning it , so you actually are not mapping the left value.

Regarding to val res = bar(foo().getOrHandle { return it }) , At first glance seems like It's not the same thing.

[Lukasz Kalnik]

Just a hint, you cannot have return within a lambda.

[Lukasz Kalnik]

It should be mapLeft ( it ) or getOrHandle ( it )

[Lukasz Kalnik]

mapLeft ( it ) and getOrHandle ( it ) both are no-ops (they don't change the incoming data)

[Lukasz Kalnik]

The difference between them is that mapLeft() returns the result of the lambda wrapped in a Left and getOrHandle() returns the result of a lambda unwrapped (as in "get what's inside the Either or handle the left result")

[Lukasz Kalnik]

Left("hello").mapLeft { it.length ) // Left(5)
Left("hello").getOrHandle { it.length) // 5

[Lukasz Kalnik]

There is also getOrElse when you just want to provide a default value and don't need to access the contents of the original `Left`:

Left("hello").getOrElse { "default text" ) // "default text"

[Lukasz Kalnik]

So I guess if your use case is to "unwrap" the value if it's a Right or provide some default value if it's a Left, and then use it inside the bar(), getOrElse() is the way to go.

[Olaf Gottschalk]

The "return it" inside the mapLeft makes the function which contains the instruction return it, the Err. Nothing gets put into res at all. So the type of res will be Either<Nothing, type of bar>